A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
Existing cochlear implant systems need to deliver electrical power from outside the body through the skin to satisfy the power requirements of the implanted portion of the system. FIG. 1 shows a typical arrangement based on inductive coupling through the skin to transfer both the required electrical power and the processed audio information. As shown in FIG. 1, an external transmitter coil 107 (coupled to the external signal processor) is placed on the skin adjacent to a subcutaneous receiver coil in the implant 108. Often, a magnet in the external coil structure interacts with a corresponding magnet in the subcutaneous secondary coil structure. This arrangement inductively couples a radio frequency (rf) electrical signal to the receiver in the implant 108, which is able to extract from the rf signal both the audio information for the implanted portion of the system and a power component to power the implanted system.
In most prior systems, the external components generally have been held in separate housings so that the external transmitter coil 107 would not be in the same physical housing as the power source or the external signal processor. The various different physical components would generally be connected by hard wire, although some systems used wireless links between separate external components. A few systems have been proposed in which all of the external components such as an external processor and a rechargeable battery could be placed within a single housing. See U.S. Patent Publication 20080002834 (Hochmair) and U.S. Patent Publication 20070053534 (Kiratzidis), which are incorporated herein by reference.
The rechargeable batteries (e.g. Lithium-Ion batteries) for such systems have conductive band materials such as aluminum and copper which are coated with battery chemistry (e.g. graphite) and are stacked on top of each other. But when such a battery is placed within a magnetic field generated by a current-carrying coil, the conductive band materials generate undesired eddy currents. Excessive eddy currents are a significant problem because they cause decreased coupling which reduces efficiency.
U.S. Pat. No. 6,067,474 by Schulman et al. teaches a coil design in the form of a long ribbon of battery electrodes. Conductive band materials are wound in a spiral (see FIGS. 7 and 9) so that the magnetic field lines generated by a coil are parallel to the conductive band materials thereby reducing eddy currents. One drawback of this approach is that in the region of overlapping conductive band materials, adjacent cathodic (46′) and anodic (48′) bands (separated by an insulator material (50′, 52′)) act as parallel plate capacitors. Such capacitance in turn may seriously increase the impedance of the coil and thereby reduce the efficiency of data and power transmission between the external coil and the implanted coil.